It is desirable for cutting inserts to be capable of high temperature operation, be resistant to wear, be shock-resistant, and be tough in order to prevent chipping or cracking during a milling or cutting operation. In the manufacture of silicon carbide components, it is known to manufacture high-temperature wear parts of sintered silicon carbide materials, as for example, as disclosed in U.S. Pat. No. 2,938,807 of Anderson. In the latter patent, .beta.-silicon carbide is described as an excellent binder, however, no diamond is incorporated in this silicon carbide technology.
A useful component of these materials would be superhard crystals such as diamond or cubic boron nitride. Their superior properties of, for example, hardness, have long been appreciated. The prior art of bonding diamond crystals usually depends on hot-press technology, and for this art reference is made to U.S. Pat. No. 4,124,401 to Lee et al, U.S. Pat. No. 4,167,399 to Lee et al, and U.S. Pat. No. 4,173,614 to Lee et al, all of which patents are assigned to the assignee of the present invention. Reference is also made to U.S. Pat. No. 4,220,455 which issued to St. Pierre, et al, and which is also assigned to the assignee of the present invention. The latter patent discloses a process for making a homogeneous diamond composite throughout an article, wherein individual crystals are coated, and Si is infiltrated into a porous preform indirectly through a wick material.
Many of the problems associated with the prior art processes for making composite cutting inserts have been overcome by the inventions disclosed in U.S. patent applications, Ser. No. 167,196 filed July 9, 1980 and Ser. No. 167,019 filed July 9, 1980 now abandoned and Ser. No. 227,228 filed Jan. 22, 1981 by John Michio Ohno. The disclosures of these applications are incorporated herein by reference. In brief, they describe bilayer diamond composites having a special binder of .beta.-silicon carbide and Si. That binder forms a matrix throughout the composite so as both to hold the diamond crystals and to unite the composite layers. Reference is also made to U.S. patent application Ser. No. 226,604 filed Jan. 21, 1981 by John Michio Ohno which discloses an improved silicon carbide composite and process for production of same. The bonded composite disclosed in the latter application comprises at least one layer containing diamond or cubic boron nitride crystals and forming a complete envelope about the core. The superhard envelope which is formed about the core defines the cutting edge of the cutting insert.
The subject invention is a further improvement in the cutting insert disclosed in the above identified patent application of John Michio Ohno and more particularly a cutting insert which uses superhard crystals in very limited critical areas, namely the cutting edges, whereby most of the side surfaces and top surface of the improved cutting insert are not covered by expensive, superhard crystal layers, thereby realizing a significant cost savings and improved performance of the subject cutting insert.